There are two aspects to the question. First, is whether the replication can be controlled inside a region and have the data of a user to live only inside that region. This is possible in theory in a couple of different ways.
If we can partition the users by region to different tables, and setup replication of all datacenters within the region, then we have achieved the boundary requirements. Some tables can be replicated to only datacenters within the region, while some other tables will be replicated cross-regions. HBase's replication model is pretty flexible in the sense that, we can do cyclic replication, etc (please read https://hbase.apache.org/book.html#_cluster_replication). If we cannot partition by table, we can still use the same table, but partition by column family (as noted above). Otherwise, we can still respect boundaries, using a recent feature called WALEntryFilter's. The basic idea would be to implement a custom WALEntryFilter which either (a) understands the data and selects which edits (mutations) to send to the receiving side (another geo-region) or (b) tag every edit with the intended regions it should hit and have the WALEntryFilter respect the tags from mutations.
The second aspect is whether you can query the whole data set from any region. Of course, if you have some data not leaving its particular geo-region, you cannot have all the data aggregated in a single DC. So the only way to access the data in whole would be to dynamically send the query to all affected geo-regions and merge the results back.
